The shadow mask used for the color cathode-ray tube is usually produced by the following process. Namely, decarbonized aluminum killed steel (hereinafter referred to as AK material) having a plate thickness of 25 .mu.m to 0.3 mm or Inver-type iron alloy (hereinafter referred to as Inver material) containing 36% of nickel having small thermal expansion coefficient is used as a substrate. Their surfaces are refined by degreasing and washing with water, and coated with a water-soluble photoresist followed by drying to make a film. Then, the mask pattern having a desired image at the position corresponding to the electron beam passing pores is brought into contact with the above film to effect exposure to ultraviolet ray. After the steps of development, hardening and heat treatment, a corrosion resistant film (resist film) with a desired pattern is formed on the substrate. After the metal part where the resist film has been removed by the development is subjected to etching using an etching solution such as ferric chloride or the like to form a number of electron beam passing pores, the remaining resist film is removed using a warm alkaline solution (for example, a 10 to 20% aqueous solution of sodium hydroxide), thereby obtaining the shadow mask. Electronic components such as IC lead frame, mesh for Vacuum Fluorescent Display, printed wiring board, and the like are also produced in the similar manner as in the production of the shadow mask using as a substrate copper, copper alloy, iron materials, 42 Nickel--iron alloy (hereinafter referred to as 42 Ni--Fe alloy, or simply 42 alloy); stainless steel, and the like.
Photoresists that are used for etching (hereinafter referred to as resists) are water-soluble negative working resists having such advantages that they are inexpensive, are not inflammable, and can be treated in aqueous solutions over the whole steps. The photoresists are prepared by adding ammonium dichromate to an aqueous solution of water-soluble polymer such as casein, polyvinyl alcohol, fish glue, and the like in an amount of 3 to 15% based on the water-soluble polymer, thereby achieving photosensitivity.
For these resists for etching, dichromate such as ammonium dichromate is used as a photosensitive agent and chromic anhydride is used as a hardener. Since these hexavalent chromium compounds are harmful, discharge of waste water containing them is strictly regulated.
The water-soluble photoresists without using hexavalent chromium (hereinafter referred to as no chromium etching resist) have been reported. They include the combination of fish glue and ferric salt, polyvinyl alcohol (hereinafter referred to as PVA) and diazo resin (Japanese Patent Publication No. Sho 56-20541 and Japanese Patent Publication No. Sho 57-6098), a water-soluble photosensitive resin containing several molar percent of N-methyl-.gamma.-(p-formylstyryl)pyridinium sulfate (Japanese Patent Application Laid-open No. Sho 55-23163), casein and water-soluble azide compound (Japanese Patent Publication No. Sho 41-7100 and U.S. Pat. No. 2,692,826), water-soluble acrylic resin having ethylenic unsaturated bonding at its side chain, anthraquinone sulfonate, and water-soluble azide compound (Japanese Patent Publication No. Sho 54-12331), and the like. However, these are not still satisfactory in view of sensitivity, resolution, and etching resistance.
As electronic components such as shadow masks and lead frames are produced in a large quantity and are highly refined, it has been desired to further improve sensitivity, resolution, and etching resistance of the resists. Especially, casein-based no chromium etching resist having practical sensitivity, resolution, and etching resistance has been strongly demanded.